Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output shaft driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns.
Some vehicles also include a transfer case which directs power from the transmission to front and rear driveshafts. Some transfer cases include shiftable gearing that allow the transfer case to operate in a low range and in a high range. The high range is typically selected for on-road driving. The low range may be selected for off-road driving or other situations in which high torque or precise speed control are desired. One disadvantage of providing this functionality in a transfer case is that it is only available in four wheel drive vehicles. Another disadvantage is the transfer case gearing is often not capable of shifting between low range and high range while the vehicle is moving under engine power.